Electric control apparatus

ABSTRACT

A motor control circuit especially suitable for low inertia motors in which the motor is energised by a succession of full power half wave pulses. For speed control the pulses are applied to the motor in forward reverse direction as the speed falls below or above the desired speed.

United States Patent 11 1 Henderson Dec. 18, 1973 [73] Assignee: Printed Motors Limited, Hampshire,

England 22 Filed: July 19, 1972 211 App1.No.:273,293

[30] Foreign Application Priority Data July 19, 1971 Great Britain 33,767/71 Sept. 9, 1971 Great Britain 42,132/71 [52] US. Cl. 318/345 [51] Int. Cl. 1102 5/16 [58] Field of Search 318/326, 327, 345

[56] References Cited UNITED STATES PATENTS 3,383,578 5/1968 Lewis 318/326 SETPRLPN BAND 1 SET INTERFACE CHANGEOVER 3,413,534 11/1968 Stringer 318/345 3,458,791 7/1969 Boice 3,450,973 6/1969 Tobey 313/345 Primary Examiner-Bernard A. Gilheany Assistant Examiner-Thomas Langer Attorney-Joseph F. Brisebois et a1.

[57] ABSTRACT A'motor control circuit especially Suitable for low inertia motors in which the motor is energised by a succession of full power half wave pulses. For speed control the pulses are applied to the motor in forward reverse direction as the speedfalls below or above the desired speed.

5 Claims, 8 Drawing Figures /v MOD s ifiW/ "li ii DRIVE AMP IN TEGRATOR PAU'INTEU 8W3 3,780,366

sum 2 or 6 SUPPLY N76 707 v /0/ INPUE (g PULSE PATENTED nan! 8 I975 SHEET 3 OF 6 Wb GR-EH5 mmwuz mo msmmmi Em SQQEEW v PAIENTED DEE] 81973 SHEET 5 0F 6 ELECTRIC CONTROL APPARATUS This invention relates to electric control apparatus. It is concerned primarily with electric control circuits for generating control or supply currents or voltages suitable for operating electric motors.

As will appear hereinafter, the invention has particular, though not exclusive application to use with low inertia electric motors, such as motors of the type in which the armature is essential laminar consisting for example, of one or more thin sheets of insulating material on which a pattern of conductors is deposited by so-called printed circuit techniques.

In the past, many methods of controlling the operation of electric motors have been proposed and used to meet a wide variety of requirements, to produce variable speed control, uniform speed control, varying torque, and other performance characteristics. Control has been effected in general to control the voltage or current applied to the motor, either continuously or by pulse methods.

The invention is concerned with an improved form of control applicable with especial advantage to the control of pulse currents applied to essentially low inertia motors.

The invention includes a motor control system, comprising means responding to an input signal, means responding to a departureof said input signal beyond limits defining a dead band to feed pulse current to said motor in a direction corresponding to the departure of said input signal beyond the upper or lower of said limits.

The invention also resides in features of control circuits, examples of which are described in embodiments of the invention, in conjunction with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of an integrating arrangement, including dead band means;

FIG. 2 is a diagram of a switching means or modulator;

FIG; 3 is a diagram of an integrator with variable leakage control, and dead band means;

FIG. 4 is a block diagram of a controller;

FIG. 5 is a more detailed diagram of the controller of FIG. 4;

FIG. 6 is a diagram of a firing circuit for a thyristor;

FIG. 7 is a diagram of a form of modulator, and

FIG. 8 is a diagram of a form of high frequency power output stage.

In the forms of the invention to be described current pulses are fed to a controlled motor. The power stage feeding the motor uses bidirectional switching means, such I as a bidirectional silicon controlled rectifier (SCR) or triac, switching half cycles of a supply. There are two types of circuit, differing in the frequency of operation, and the duration of the pulses fed to the motor. For example, in the one type of circuit current pulses of 10 m.sec can be used, corresponding to SOHZ; in the other pulses of 200 psec. at a frequency of 3.3 KHZ, can be used. This high frequency pulse operation is feasible when the controlled motor has a sufficiently low inertia; a typical motor might have a low resistance armature of say 1 ohm and an inductance of 50 pH, giving a time constant of 50 psec.

One form of the present invention is a process controller in which the output information consists of a series of steps, each step being at maximum output power; the stepping rate is adjustable, and is a function of amplifier input. Each step consists of one half cycle of a reference frequency, either internally generated, or derived from an external supply. The polarity of the input signal determines whether positive or negative half cycles of the reference frequency are produced.

In this process controller, the open loop characteristics are similar to the characteristics of a conventional process controller; before describing the transfer characteristics, it is convenient to explain certain terms, as used herein, as follows:

The proportional band is the range of input or error signal, in which proportional action, that is, action which causes the output device to move a distance proportional to the error signal at maximum speed, takes place. The width of the proportional band is inversely related to the gain of the controller, so that a controller with a wide proportional band has a low gain.

Integral action is the action of a controller when the output device moves at a velocity proportional to the error.

Integral time is the time taken for the output device to move a distance under integral action, equivalent to the distance moved under proportional action.

Derivative action is the action that causes the output device to move a distance proportional to the rate of change of error, or alternatively the (proportional band/controller gain) ratio varies with the rate of change of error.

A discontinuous action process controller is one in which the controller drives a motorised device in a series of steps, each at maximum output power. The integral action, (as defined above), is achieved by varying the rate at which these steps occur. The position that the device takes up is always a function of the time for which the output has been applied and not its analog value; the output device thus integrates the output signal with respect to time.

A continuous action controller differs from the discontinuous controller in that it produces a continuous analog output voltage or current which can be used to drive a variety of devices. The position that the device takes up is always directly related to this output signal. Integration in a discontinuous process controller differs from the continuous action controller in that it takes place in the output device. Integration in the continuous controller takes place inside the controller.

The open loop response of the controller to a step input is the change of position of the output device to a step voltage applied to the amplifier input terminal. By way of example, the controller to be described responds to a step input in two distinct sequential modes: In the first mode the response is proportional, and motor will run at maximum speed for a period proportional to the magnitude of the step. In the second mode the response is integral, and the motor will run at a speed proportional to the magnitude of the step.

With a continuously acting controller the proportional action is instantaneous within practical limits. Derivative action causes the output device to move a maximum distance during the proportional period in response to a step input. This distance is reduced for a slowly changing input voltage.

Negation is the operation of reducing the signal at the input of an amplifier or other device by the action of a negative feedback voltage or current.

It is also convenient to describe components of a complete system before describing the system as a whole.

FIG. 1 is a block diagram of an arrangement comprising an operational amplifier 10, having negative feedback provided by a second amplifier 11 which with a feedback capacitor 12 operates as an integrator. The amplifier has input resistor 13 and feedback resistors l4 and 15. The amplifier 10 will give an output when subjected to a step input, and this output will continue to exist until the integrator has had time to charge up and negate the input signal.

The period for which the output exists is thus a function of the integrator time constant and the magnitude of the step input, and, assuming that the integrator produces a linearly changing integration output, the time duration of the output for a fixed integrator time constant is proportional to the magnitude of the step input.

In a practical system, a dead band exists, so that an input'signal within the dead zone will not produce an output change. In such a case, integration will occur only when the output from the amplifier exceeds predetermined positive or negative levels which define the dead band, and integration will cease when the input step has been negated to lie within this range. The predetermined levels are represented, in FIG. 1 by positive and negative responsive switches 16 and 17 in the feedback path.

If the integrator is perfect and the integrated voltage is not diminished by leakage the system will remain in this condition, with this value of output, indefinitely. However, if a leakage path, as presented by resistor 18 in FIG. 1 is provided across the integrator after a time interval depending upon the leakage the integrator ceases to negate the input and the amplifier output to exceed the predetermined level and integration will recommence. In this manner the system of FlG. 1 will continue to repeat at a fixed rate determined by the various fixed constants in the circuit including integration rate, leakage, amplifier gain and switching hysteresis. The negative and positive switch hysteresis is important', without it, the integrator would need to change by only a very small voltage to re-establish integration and thus cause an oscillatory condition at high frequency.

Switching devices used in the controller can advantageously be triacs, which can be gated by either positive or negative pulses. Two triacs are shown at 19 and 20 in FIG. 2, with a common resistor 21, one gated by a positive voltage through diode 22 and the other gated by a negative voltage through diode 23, the negative gated triac 20 being fed from the negative half wave SOI-lz supply through diode 24 and the positive gated triac 19 is fed from positive half wave 50 Hz through diode 25, hence the triac which conducts will depend on the polarity of the gate signal. The output from the demodulator is obtained from the voltage across resistor 21. It is highly desirable that a triac will be switched on at the beginning of the respective half cycles. This can be achieved in the manner shown in FIG. 2 by shorting the gate signal to zero voltage except during the required gating period, ie 0-10 or l80-l90 as the case may be, of the supply frequency. The gate electrodes are short circuited by transistors 26, 27, the base electrodes of which are switched by applying to them the alternating supply through rectifiers 28, 29 with capacitor-resistor filters 30, 31.

If the rate of charge of the capacitor 12 of the integrator is varied during this initial proportional period without affecting the integral action, then a third term has been introduced, in effect giving the ability to determine for how long the output device will run at maximum speed after the application of a step input. This integration speedup facility can be achieved in the manner shown in FIG. 3, by using two zener diodes 32, 33 connected back to back, and selected to cease conducting only when the negation signal is close to that required for complete negation. The zener diodes 32, 33 in series opposition, are connected across the output of amplifier 10, in series with a potentiometer 34, the tapped voltage being applied to the input of amplifier 11. FIG. 3 shows an additional amplifier 35 which it may be convenient to include.

During the conduction period the integrator charge rate is increased by feeding the signal from potentiometer 34 into the summing junction of amplifier l1, and potentiometer 34 enables this additional charging current to be set to the required value. I

An important feature of the integrator is that the output rate of change of voltage is not a function of the setting of the leakage resistor 18. This means that one half cycle of supply frequency is always sufficient to move the integrator negation voltage through the hysteresis of the triac gate requirements and thus only one half cycle is generated before the triac switching voltage is negated into the dead zone.

A block diagram of a process controller suited to low frequency operation is shown in FIG. 4. Input error signals are applied to terminals 40 and fed to a summing amplifier 41 of unity gain; the amplifier may incorporate a noise filtering capacitor. The amplifier output voltage is fed to two independent variable gain amplifiers 42, 43 of which the first is an interface changeover amplifier 42.

The output of amplifier 42 is fed to positive and negative error detectors 44, 45. When the output signals exceed a predetermined value and depending on output polarity, the interface changeover relay 46 is operated. The effect of this relay as will appear is to route the controller output to a reduced voltage supply, when the error in the system falls below a predetermined level.

The second amplifier 43 is a proportional band amplifier, also fed from the summing amplifier; the gain of this amplifier sets the system proportional band. The output of amplifier 43 is fed to the inverting input of the error and integration summing amplifier 47.

The integrator output is fed to the one input of this amplifier; to keep the sense correct, the output is fed to the non-inverting input.

The output of the amplifier 47 is applied to the noninverting input of the driver amplifier 48 for modulator l9, and also to two zener diodes 50, 51 connected back to back; the diodes are returned to a point of zero voltage through a potentiometer 52, as in FIG. 3, the tap 53 of the potentiometer is connected to one input of integrator 54, through a fixed resistor 55 which contributes to the time constant. Potentiometer 52 sets the proportional negation, or derivative term, as described above.

The modulator drive amplifier 48 has a fixed low gain. The modulator is fed with an alternating, conveniently 50 Hz or mains frequency, supply and the modulating triacs included in the modulator are set to switch at a fixed positive or negative voltage at the input of the modulator, for example 1': 3 volts. If the modulator amplifier has a gain setting of 3 times this corresponds to i 1 volt at the output of the error and integration summing amplifier 47. The two zener diodes 50, 51 switch at say, i 5 volts and are effective only when the modulator drive amplifier is saturated. As in FIG. 2, the two triacs in the modulator 49 are arranged so that one will switch on and conduct the positive half wave of the applied alternating supply when the. gate signal is positive and the other will conduct the negative half wave when the gate signal is negative. A third triac is used to combine the positive and negative half wave pulses from each triac and to produce 21 voltage with respect to the zero voltage line. The triacs are prevented from firing except at the desired period at the beginning of the respective half cycles by the two transistors used to impose a short circuit on the output from the modulator drive amplifier.

The integrator 54 is fed with two signals, the first as described above, from the two zeners 50, 51, the second signal being the positive or negative half wave pulse generated by the modulator 49. These input signals are combined and the integrated signals are fed back through resistor 57 to the non-inverting input of the error and integration summing amplifier 47. A variable resistor 58 is connected across the integrator capacitors 59, 60, conveniently two electrolytic capacitors connected in opposite polarity. This resistor 58 will have two functions: it establishes the gain of the integrator, and thus the maximum output voltage, and it sets the capacitor leakage rate. The value of error signal against which integration is effective, and also the rate at which the integration capacitors leak for a given error, are thus set by resistor 58. I

The modulator output is applied to a pulse shaper 61. The input of this pulse shaper includes PNP and NPN transistors switching either the positive or negative half wave output from the modulator. The leading edge of the resultant square wave is differentiated and fed over the contacts of the interface changeover relay to one of two thyristor pulse firing transformers.

Two interface circuits 62, 63 are incorporated with a pulse routing relay 46, as described above. The pulse shaper output is controlled by the contacts 64 of relay 46 to apply the output to one or other of the two interface circuits 62, 63, which control respectively triacs 65, 66 which apply a lesser or a greater voltage from windings of a supply transformer 67 to motor 68.

The arrangement of FIG. 4 is shown in somewhat more detail in FIG. 5. In the two figures like parts bear like reference numerals. It is considered unnecessary to describe FIG. 5 in complete detail, in view of the foregoing description.

In FIG. 5 the triacs of the modulator are shown at 68 and 69, with the third triac 70. The pulse shaper 61 in cludes the input transistors 71, 72 and the output transistor 73.

In the interface circuits the input pulse from the pulse shaper is applied, through contacts 64, and pulse transformer 75 to thyristor 76 having a capacitor 77 shunted by a resistor 78 in its cathode circuit. The anode is connected through a resistor 79 to a capacitor 80 supplied through resistor 81 from a bridge rectifier 82. An additional capacitor 83 shunts the rectifier bridge output. For output pulses from pulse shaper 61 of the other polarity a similar circuit is provided including transformer 86, thyristor 87, capacitor 88, resistor 89, resistor 90.

On receiving an input pulse the relevant thyristor 76 or 87 switches on and charges up capacitor 77 or 88 until the current flowing falls below the necessary holding current, the thyristor then switches off, the supply being limited by the time constant of the circuit 80, 81.

The voltage developed across capacitor 77 or 88 is connected through diac 92 or 93 to the gate of triac 94 or 95 and pulses are developed until the capacitor 77 or 88 is discharged. It is to be observed that with a motor of the low inertia, low inductance type, such as one using a printed circuit armature, it becomes feasible to use the back emf generated by the armature as an indication of speed motion of the armature, to stop further supply of the armature when for example a driven member reaches a positive stop. With a conventional motor the winding inductance is too great to allow the back emf to fall rapidly and substantial overrun may result. Hence, by using, for example, a thyristor and a resistor on the negative or succeeding half cycle of the applied voltage a detecting voltage is developed, which can be used for control.

FIG. 6 is also a circuit detail of an arrangement capable of generating a train of high power pulses in response to single low power pulse, which can be used for the triggering of thyristors. An input pulse is applied to the primary winding of pulse transformer 101. The secondary winding of this pulse transformer is connected between gate and cathode of thyristor 102, the anode of which is fed through resistor 103 to the junction of the cathodes of diodes 104, and 105. The anodes of diodes 104, 105 are connected to the windings 106, 107 of a centre-tap transformer 108 so that the voltages on the two anodes are out of phase and of the same amplitude. The centre tap of windings 106, 107 is taken to a point at zero potential. The cathode of thyristor 102 is connected to zero potential through capacitor 109 shunted by resistor 110. A diac 111 with a series limiting resistor 112 is connected between the cathode of thyristor 102 and a resistor 113. The other end of resistor 113 is connected to zero potential.

An input pulse at 100 switches on thyristor 102 and capacitor 109 charges at a rate determined by resistor 103. The voltage across capacitor 109 rises to a value at which the diac 111 switches on, and capacitor 109 discharges through the diac into resistor 113. The voltage across this resistor is applied in this example, to the gate of a triac 114 which switches on and further discharges capacitor 109.

The voltage across capacitor 109 falls below the voltage necessary to sustain the current in the diac, but thyristor 102 remains in the on state by the action of resistor 1 l0 and capacitor 109, until the current falls below the holding value. The voltage across capacitor 109 again increases until diac 111 again switches on. This cycle is repeated for part of the half cycle of voltage from transformer 108 at the end of which period thyristor extinguishes thus producing a number of pulses in response to the input pulse.

Circuits can be devised as described herein to give a highly satisfactory degree of control. For integral action, the output pulse rate can be varied with reference to the input signal by adjusting one pre-set control. At minimum setting the output pulse rate will be at maximum, that is the reference supply frequency, when the input to the controller just exceeds the pre-set electrical dead zone. At maximum setting the output pulse rate will not achieve maximum rate until the input signal is at a pre-set maximum value. The relation between the input signal level and pulse output rate can be made linear, and the range of integral times available, because of the flexibility of the design, allows it to be used as a fast position servo, or as a valve positioner and controller, in a process where lags of minutes may be present.

When the supply is used as a reference frequency a bidirectional thyristor is used to switch either negative or positive half cycles of the supply into the output load. The number of half cycles switched per unit period is a function of the input signal level and integral action setting. The printed motor is particularly suit able as an output load for the controller, and when used with a triac, high power can be switched at low cost. At higher reference frequencies alternative switching semiconductors are used. The maximum switching frequency will be determined by motor losses, duty cycle, and the acceptable motor efficiency.

In many servo applications not requiring continuous control, the controller described, with a printed motor, has advantages, for example fast acting position servos can be built without recourse to tacho generators since incremental techniques give flexible approach speeds. Overshoot can be eliminated and mechanical backlash absorbed on approaching balance slowly from one direction only. Maximum torque is developed at all times. The inertia and lag problems familiar in DC systems can be resolved by setting the integral action control to optimise approach speeds. Triac operation enables high powers to be controlled with minimum cost and space. Units can be encapsulated, with externally available means for controlling the proportional band and integral time.

The resolution of the system is determined by the time duration of one output increment and the full travel time of the output device. For example, an output device with a full travel time of 2 seconds and an incremental pulse length of 10 m/s would have a maximum resolution of approximately one part in two hundred. A big improvement in performance can be obtained by automatically reducing the size of increment within the region of system balance.

This is achieved by using the two output triacs and a tapped supply transformer, with the logic circuit rerouting the gate pulses to the second triac connected to a lower voltage supply, at a pre-set point determined by system dynamics. The pre-set adjustment of this point can be by a potentiometer mounted on the front panel of the unit.

To overcome the limitations of the relatively low frequency steps of the system using a 50112 reference frequency, it is possible to use an internally generated reference frequency. This can be any value, for example, a 1000 Hz system has been found practicable.

FIG. 7 is the circuit diagram of an arrangement which can be used to provide signals positive and negative with respect to zero for use at high frequency, in place of the triacs described with reference to FIGS. 2 or 5.

Two triacs 130, 131 can be triggered by input pulses applied through the oppositely polarised diodes 132, 133; the triacs are fed through diodes 134 and 135 and resistors 136, 137. The voltages across these resistors are applied to the bases of transistors 138, 139 of oppo- 6 site conductivity types, from the collector load resistors 140, 141 of which outputs are obtained at 142, 143.

FIG. 8 is a circuit diagram of a power stage of a control circuit, suitable for use at high frequency.

The circuit shown comprises terminals and 151 for connection to a unidirectional current source, such as a rectified alternating current supply; this supply is used to feed a motor 152. The feed to the motor is controlled by a series of silicon controlled rectifiers, or thyristors, or equivalent semiconductor switching devices, in conjunction with a capacitor 153 which, by the appropriate switching operation of the thyristors is charged and discharged through the motor winding.

The triggering potentials for the thyristors are provided by a means of a trigger control unit 154, which as described above operates to produce triggering pulses, but at relatively high frequency, so that the current fed to the motor is likewise in the nature of rapidly occurring current pulses.

There are eight thyristors shown in the circuit, bearing references 161 and 168 respectively. Of these 161 and 162 are connected in series between terminals 150 and 151, and 163 and 164, also in series, are similarly connected. The motor 152 is connected between the junction point A of 161 and 162, and the junction point B of 163 and 164. 165 and 166, are connected, in reversed polarity in parallel, between point A and one terminal of capacitor 153; the other terminals of capacitor 153 is connected to terminal 151 of the supply. 167 and 168 are connected in reversed polarity in parallel, between point B and the one terminal of the capacitor 153. The trigger electrodes of all the thyristors, marked a to d are correspondingly marked to outputs a to d of the control unit 154.

Initially, 161 and 168 are switched on and current will flow from terminal 150, through 161 the motor 152 and 168 to the capacitor 153 so that the capacitor will charge and the charging current will energise the motor. The charging current will diminish exponentially, until 161 and 168 switch off, due to lack of holding current. Thyristors 166 and 164 are then switched on, so that the charge on the capacitor is discharged, the discharge current passing in the same direction through the motor, and thus sustaining the motor energisation. The cycle is then repeated.

The motor can be made to run in the reverse direction by first switching on 163 and 165, to charge the capacitor, and then 167 and 162 to discharge the capacitor.

The repetition frequency of operation can be high, a frequency of up to 4 KHz having been found practicable. The capacitance of capacitor 153 is chosen with reference to the repetition frequency and the resistance of the motor windings. The supply voltage at terminals 150, 151 is chosen in relation to the nominal motor voltage. In practice, the voltage can lie for example in the range of 20 to 230 volts; in the case of a motor of 1 ohm resistance, the capacitor C had a capacitance of 50 pF.

I claim:

1. A motor control system comprising:

i. a first operational amplifier,

ii. means for applying to the input of said first operational amplifier a first input signal the polarity and magnitude of which is indicative of a departure as to sense and magnitude of a controlled quantity,

iii. a power supply control stage, said power supply control stage having a supply input and a control input,

iv. a control voltage source including said first operational amplifier for supplying a control input voltage to said control input of said power supply control stage,

v. a power supply source for supplying voltage pulses to said supply input of said power supply control stage, said pulses being at constant frequency and of constant pulse width, whereby saidpower supply control stage is caused to produce output pulses of constant duration and amplitude and of a number controlled in accordance with the output of said first operational amplifier,

vi. a second operational amplifier,

vii. integrating means for producing an integration voltage which is an integral of the output voltage pulses from said power supply control stage,

viii. means including said second operational amplifier for applying to the input of said first operational amplifier the integration voltage to negate the effect of said first input signal to said first operational amplifier, and

ix. means for supplying current to said motor in accordance with the output pulses from said power supply control stage.

2. A motor control system as claimed in claim 1, and comprising circuit means for varying the integration rate of the said integrating means in accordance with the output of said first operational amplifier, to provide derivative control, said circuit means including a capacitor connected between an input and an output of said second operational amplifier whereby said second operational amplifier functions as the said integrating means, a voltage limiting means in series with a resistor connected across the output of the first operational amplifier, and means for feeding a voltage developed across said resistor to the said input of the second operational amplifier.

3. A motor control system as claimed in claim 1, and wherein said power supply source supplies to the power input of said power supply source an alternating voltage, said power supply source including switching means to select half cycles of the applies alternating voltage, switch control means for controlling said switching means in accordance with the output of the operational amplifier, said switching means including a first circuit comprising a first triac in series with a first diode, a second circuit comprising a second triac in series with a second diode, said first and second circuits being connected in parallel across said power supply with said diodes in opposite polarity, means for applying an input applied to the trigger electrodes of said triacs through third and fourth diodes to trigger said triacs alternatively.

4. A motorcontrol system as claimed in claim 1, and comprising a direct current source, and a power control stage for controlling supply of current from said direct current source to said motor, said direct current source having first and second terminals, said power control stage comprising a capacitor, and first, second, third and fourth switching devices, said motor capacitor and four switching devices being connected in a circuit network, to present a first circuit from said first terminal through said first switching device, said motor, said second switching device and said capacitor, means responsiveto said power supply control stage to switch on said first and second switching devices whereby said capacitor is charged and the charging current flows through said motor, said circuit presenting also a second circuit including said capacitor, said third switching means, said motor and said fourth switching means, means responsive to said power supply control stage to switch on said third and fourth switching means whereby the charge on said capacitor is discharged through said motor.

5. A motor control system as claimed in claim 4, and comprising fifth, sixth, seventh and eighth switching devices, said fifth switching device being connected in series with said first switching device across the terminals of said supply, said sixth switching device being connected in series with said fourth switching device across the terminals of said supply, said seventh switching device being connected in parallel with said second switching device and said eighth switching device being connected in parallel with said third switching device. 

1. A motor control system comprising: i. a first operational amplifier, ii. means for applying to the input of said first operational amplifier a first input signal the polarity and magnitude of which is indicative of a departure as to sense and magnitude of a controlled quantity, iii. a power supply control stage, said power supply control stage having a supply input and a control input, iv. a control voltage source including said first operational amplifier for supplying a control input voltage to said control input of said power supply control stage, v. a power supply source for supplying voltage pulses to said supply input of said power supply control stage, said pulses being at constant frequency and of constant pulse width, whereby said power supply control stage is caused to produce output pulses of constant duration and amplitude and of a number controlled in accordance with the output of said first operational amplifier, vi. a second operational amplifier, vii. integrating means for producing an integration voltage which is an integral of the output voltage pulses from said power supply control stage, viii. means including said second operational amplifier for applying to the input of said first operational amplifier the integration voltage to negate the effect of said first input signal to said first operational amplifier, and ix. means for supplying current to said motor in accordance with the output pulses from said power supply control stage.
 2. A motor control system as claimed in claim 1, and comprising circuit means for varying the integration rate of the said integrating means in accordance with the output of said first operational amplifier, to provide derivative control, said circuit means including a capacitor connected between an input and an output of said second operational amplifier whereby said second operational amplifier functions as the said integrating means, a voltage limiting means in series with a resistor connected across the output of the first operational amplifier, and means for feeding a voltage developed across said resistor to the said input of the second operational amplifier.
 3. A motor control system as claimed in claim 1, and wherein said power supply source supplies to the power input of said power supply source an alternating voltage, said power supply source including switching means to select half cycles of the applies alternating voltage, switch control means for controlling said switching means in accordance with the output of the operational amplifier, said switching means including a first circuit comprising a first triac in series with a first diode, a second circuit comprising a second triac in series with a second diode, said first and second circuits being connected in parallel across said power supply with said diodes in opposite polarity, means for applying an input applied to the trigger electrodes of said triacs through third and fourth diodes to trigger said triacs alternatively.
 4. A motor control system as claimed in claim 1, and comprising a direct current source, and a power control stage for controlling supply of current from said direct current source to said motor, said direct current source having first and second terminals, said power control stage comprising a capacitor, and first, second, third and fourth switching devices, said motor capacitor and four switching devices being connected in a circuit network, to present a first circuit from said first terminal through said first switching device, said motor, said second switching device and said capacitor, means responsive to said power supply control stage to switch on said first and second switching devices whereby said capacitor is charged and the charging curreNt flows through said motor, said circuit presenting also a second circuit including said capacitor, said third switching means, said motor and said fourth switching means, means responsive to said power supply control stage to switch on said third and fourth switching means whereby the charge on said capacitor is discharged through said motor.
 5. A motor control system as claimed in claim 4, and comprising fifth, sixth, seventh and eighth switching devices, said fifth switching device being connected in series with said first switching device across the terminals of said supply, said sixth switching device being connected in series with said fourth switching device across the terminals of said supply, said seventh switching device being connected in parallel with said second switching device and said eighth switching device being connected in parallel with said third switching device. 